1. Field of the Invention
The present invention relates to a computer numerical control (CNC) machine center, and particularly to a CNC machine center using a screw rod having positive thread and counter thread respectively in association with two nuts respectively secured on a tool holder assembly and a worktable assembly to simultaneously shift the tool holder assembly and the worktable assembly in opposite directions along at least one coordinate axis, so as to significantly increase the relative shifting speed between the tool holder assembly and the worktable assembly, to minimize non-cutting time of a CNC machine center.
2. Description of Related Art
Generally, in order to accomplish a purpose for cutting a workpiece by a tool, a computer numerical control (CNC) tool machine is equipped with facility for feeding the tool toward the workpiece during a cutting operation period and with facility for changing and adjusting the relative positions between the tool and the workpiece during a non-cutting operation period. In the cutting operation, the speed for feeding the tool toward the workpiece is determined by the characteristics of the material to be machined, and thus shall not be changed or increased arbitrarily. However, if the speed for shifting or adjusting the relative positions between the tool and the workpiece during the non-cutting operation period is increased, the overall machining speed and efficiency of the machine center will be greatly improved.
FIG. 1 illustrates a conventional CNC machine center (M/C) having a worktable T engaging a translation seat T1 by virtue of a nut (not shown) in association with a screw rod R, so that the worktable T is reciprocatable along X-axis on the translation seat T1, when a servo motor M rotates the screw rod R. The translation seat T1 engages the frame foundation F by an additional screw rod/nut mechanism, so that the translation seat T1 is reciprocatable along Y-axis on the frame foundation F. Similarly, a tool holder B is disposed on an upright stand FV by a nut N and a screw rod R, so that the tool holder B is reciprocatable along Z-axis on the upright stand FV. Therefore, the relative positions in three coordinate axes between the tool holder B and the worktable T are all adjustable to facilitate the machining operation.
Referring to the conventional manner of applying a screw rod/nut mechanism, e.g., the transmission mechanism including the screw pair of the screw rod R and the nut N illustrated in FIG. 1, to change the relative positions between a tool holder and a worktable, if one intends to increase the speed for the tool holder to get closer to or to depart away from the worktable, he can only accomplish this purpose by increasing the rotation of the screw rod by using a higher speed servo motor. It is, however, found that none of the existing servomotors available in marketplace would satisfy this need for providing a satisfactory shifting speed between the tool holder and worktable during the non-cutting operation period.
Especially, when the conventional screw rod/nut mechanism is applied to reciprocate the tool holder along a vertical axis, e.g. the tool holder B reciprocated along vertical Z-axis by the screw rod R in association with the nut N of the conventional CNC machine center illustrated in FIG. 1, the load of the servo motor M to rotate the screw rod R for upwardly shifting the tool holder B is significantly greater than the load of the servo motor for downwardly shifting the tool holder B, due to the weight of the tool holder B. This not only reduces the service life of the servomotor, but also is disadvantageous to the control of the operational speed. To overcome such problem as to the servo motor bearing different loads when driving a screw rod to upwardly and downwardly shift the tool holder B, the conventional art applies a chain, wire or the like to draw a balance weight, to balance the weight of the tool holder B, and to avoid the problem that the servo motor has different loads when upward and downward shifting the tool holder B. Though this is feasible and practicable, it makes a tool machine complicated in construction, bulky and cumbersome, and disadvantageous for manufacture and maintenance.
Referring to the conventional CNC machine center illustrated in FIG. 1, the tool holder B is shifted upward or downward when the servo motor M clockwise or counterclockwise rotating the screw rod R. Such an operation manner and construction of applying a servo motor M in association with a screw rod/nut mechanism to upward and downward shift a tool holder B, when electric power is unexpectedly off or the servo motor is not energized, the weight of the tool holder B creates a torque to rotate the freely rotatable screw rod R, and is gradually moved downward by the weight of the tool holder B. This would subsequently result in that the tool holder collides with and damages the workpiece, worktable, or the other parts of the machine.
An object of the present invention is to provide a CNC machine center, in which the relative shifting speed along at least one coordinate axis between the tool and the workpiece is the double of the relative shifting speed of the conventional one applying a screw rod/nut mechanism to simply shift either the tool or the workpiece. The present invention applies a screw rod having two sections of opposite threads, namely positive thread and counter thread, in association with two nuts separately secured on a tool holder assembly and a worktable assembly to accomplish the relative movement or shifting of the tool and the workpiece, so that the non-cutting operation time of a CNC machine center is significantly reduced, and the overall machining speed and efficiency are improved.
A further object of the present invention is to provide a CNC machine center, in which a tool holder assembly and a worktable assembly are reciprocatably disposed along a vertical axis by virtue of a screw rod having two sections of opposite threads, namely positive thread and counter thread, respectively, in association with two nuts respectively secured on the tool holder assembly and the worktable assembly, so that a servo motor, when rotating the screw rod clockwise or counterclockwise to shift the relative position along the vertical axis between the tool holder assembly and the worktable assembly, is of a substantially identical load, without using any additional balance weight.
A still further object of the present invention is to provide a CNC machine center that can avoid the tool holder to collide with the workpiece or the machine, when the electric power is unexpectedly off or the servomotor is not energized.
Additional objects, operational principle, construction, advantages, construction, and features of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements.